1. Field of the Invention
The present invention relates to a vibrator having a vibration mode of in-plane vibration in a main surface and a vibration mode of out-of-plane vibration in a direction perpendicular or substantially perpendicular to the main surface, and a vibrating gyroscope detecting an angular velocity applied to a vibrator around a rotation axis perpendicular or substantially perpendicular to the vibration direction of each of the two vibration modes.
2. Description of the Related Art
A vibrating gyroscope detecting an angular velocity includes a vibrator having a first vibration mode (drive vibration mode) of vibrating along a drive axis perpendicular to a rotation axis and a second vibration mode (detection vibration mode) of vibrating along a detection axis perpendicular to the rotation axis and the drive axis. When the vibrator vibrating in the drive vibration mode rotates around the rotation axis, a Coriolis force along the detection axis is applied to the vibrator. When the Coriolis force is applied, the vibrator vibrates in the detection vibration mode. The vibration amplitude of the detection vibration mode becomes an amplitude according to the magnitude of the angular velocity of a rotational movement, in other words, the magnitude of the Coriolis force occurring owing to the angular velocity of the rotational movement. Therefore, by detecting the vibration amplitude of the detection vibration mode, it may be possible to detect the angular velocity of the rotational movement.
The structure of a vibrator used for a vibrating gyroscope varies. A type of vibrator is configured as a tuning fork-type vibrator including two cantilever beams (See, for example, International Publication No. WO2008/010336).
FIG. 1A is a plan view of a vibrating gyroscope 111 including a tuning fork-type vibrator of the related art, and FIG. 1B is the partial cross-sectional view thereof.
The vibrating gyroscope 111 includes a vibrator 101 serving as the tuning fork-type vibrator. The vibrator 101 includes leg portions 101A and 101B, a supporting portion 101C, and a base portion 101D. Each of the leg portions 101A and 101B is formed into a meander shape. One end side of each of the leg portions 101A and 101B is joined to the base portion 101D, and serves as a fixed end. The other end side of each of the leg portions 101A and 101B serves as a free end. The supporting portion 101C is disposed between the leg portions 101A and 101B, and formed so as to extend from the base portion 101D in the same direction as the leg portions 101A and 101B. The vibrator 101 includes a dielectric film 102, a piezoelectric body film 103, an electrode film 104, a substrate 105, and a common electrode 106. The dielectric film 102 is formed in the top surface of the substrate 105. The piezoelectric body film 103 is formed in the top surface of the dielectric film 102. The electrode film 104 is formed in the top surface of the piezoelectric body film 103. The common electrode 106 is formed in the bottom surface of the substrate 105, and connected to a ground. In the supporting portion 101C, the vibrator 101 is supported by a supporting substrate not illustrated.
The electrode film 104 includes electrodes 104A to 104C. The electrode 104A includes line-shaped portions individually following the leg portions 101A and 101B and the supporting portion 101C, and a portion connected to these line-shaped portions and formed in the base portion 101D. The electrode 104B is formed into a line shape so as to lead from an end portion of the leg portion 101A, which serves as the free end, to an end portion of the supporting portion 101C through the upper portion of the base portion 101D. The electrode 104C is formed into a line shape so as to lead from an end portion of the leg portion 101B, which serves as the free end, to an end portion of the supporting portion 101C through the upper portion of the base portion 101D.
The electrodes 104A to 104C, the common electrode 106, and the piezoelectric body film 103 configure an electromechanical conversion element. In the vibrator 101, a vibration is excited in such a manner that the end portions of the leg portions 101A and 101B, which serve as the free ends, move away from or come close to each other and are opened or closed. In the vibrating gyroscope 111, such a vibration mode is used as the drive vibration mode. In the vibrator 101, a vibration is excited in which the leg portions 101A and 101B perform bending vibration in the thickness directions thereof. In the vibrating gyroscope 111, such a vibration mode is used as the detection vibration mode. The vibrating gyroscope 111 detects the angular velocity, using the fact that when, in a state where a vibration occurs in the drive vibration mode, an angular velocity is applied to the vibrating gyroscope 111 around an axis parallel to the leg portions 101A and 101B, the vibrator 101 vibrates in the detection vibration mode due to the Coriolis force.
It is desirable that a detection sensitivity for an angular velocity is high in a vibrating gyroscope. In general, so as to enhance the detection sensitivity for the angular velocity in a vibrating gyroscope utilizing a tuning fork-type vibrator, it is necessary to adequately set the resonant frequency of a vibrator. Since the resonant frequency of the tuning fork-type vibrator is inversely proportional to the square of the length of a leg portion (beam), the resonant frequency becomes significantly high when the tuning fork-type vibrator has been miniaturized. In addition, the detection sensitivity becomes low as the resonant frequency becomes high. Therefore, in the vibrating gyroscope 111, by arranging the leg portions 101A and 101B in the meander shapes, it may be possible to maintain a long leg portion (beam) even if the vibrator 101 is miniaturized, and the resonant frequency of the vibrator 101 is prevented from becoming high.
Here, a relationship between the resonant frequency of the vibrator and the detection sensitivity for an angular velocity will be described.
The detection sensitivity for an angular velocity may be expressed as a value proportional to the product of the maximum value of the Coriolis force applied to the vibrator and a detected voltage (hereinafter, referred to as a detection efficiency) output per 1 N (Newton) of the Coriolis force. The maximum value of the Coriolis force may be expressed as the product of the mass of the vibrator, the maximum velocity of the displacement of the vibrator in the drive vibration mode, and an angular velocity applied to the vibrator. Accordingly, the detection sensitivity for the angular velocity may be expressed as a value proportional to the product of the detection efficiency, the mass of the vibrator, and the maximum velocity of the displacement of the vibrator in the drive vibration mode.
The detection efficiency, the mass of the vibrator, the maximum velocity of the displacement of the vibrator in the drive vibration mode, and so forth have correlations not only with the detection sensitivity but also with the thickness of the vibrator, a width dimension, a stiffness property, a resonant mode, and the resonant frequency thereof.
In recent years, the miniaturization of a vibrating gyroscope has been strongly desired. In general, when a vibrator becomes small, the resonant frequency of the vibrator becomes high. Therefore, when a vibrating gyroscope including a small vibrator has been installed in a digital camera or the like, a difference between the resonant frequency of the vibrator and the frequency of a hand movement becomes large. Therefore, a sensitivity for the hand movement or the like becomes low in some cases.
Therefore, the vibrator is caused to have a specific structure or the vibrator is caused to vibrate in a specific vibration mode, and hence, even if the vibrator is small, it may be possible to prevent the resonant frequency of the vibrator from being increased.
Furthermore, so as to improve the drift characteristic of the vibrating gyroscope, it is necessary for both of the drive vibration mode and the detection vibration mode to share a common node.
By supporting the vibrator using the common node, it may be possible to prevent a vibration from leaking from a supporting portion supporting the vibrator or prevent an undesired vibration from propagating from the outside, and it may be possible to obtain a good drift characteristic.
In the vibrator 101 including the leg portions 101A and 101B having the meander shapes, since end portions of the leg portions 101A and 101B are fixed ends, the end portions being joined to the base portion 101D, it is necessary to provide the base portion 101D whose stiffness property is high. Therefore, the base portion 101D turns out to account for a large percentage of the area of the vibrator 101, and even if the leg portions 101A and 101B have the meander shapes so as to make the vibrator 101 smaller, the ability to lower the resonant frequency of the vibrator 101 has been inevitably limited due to the base portion 101D.